In recent years, an increasing number of flat panel color display tubes have been developed. There has been a problem that pincushion distortion at the top, bottom, left and right are likely to increase when a deflection yoke for a conventional color display tube with a curved panel is used in a color display tube with a flat panel. Accordingly, efforts have been made to solve these problems.
In the problems mentioned-above, the pincushion distortion at the top and bottom is corrected generally by means of adding magnets to the above and below part of the deflection yoke at the screen side. This utilizes the effect that strong pincushion magnetic fields generated by the magnets draw electron beams in a vertical axis direction. Consequently, with respect to mis-convergence on the vertical axis (YH), a pincushion-shaped residual mis-convergence is likely to occur in a self-convergence system, as shown in FIG. 10. Along with the development of flat panels, there is a tendency to increase the pincushion distortion at the top and bottom. Therefore, the magnetic field generated by the magnet is further strengthened for correcting this distortion, thus increasing the YH pincushion-shaped residual mis-convergence even more.
In order to solve such a problem, four-pole coils for generating a four-pole magnetic field and YH correction circuits are added to the deflection yoke on the electron gun side, so that a vertical deflection current is supplied to them, thereby performing a YH correction. The principle of this YH correction is as follows. Before electron beams enter a main magnetic field generated by the deflection yoke, the four-pole magnetic field generated by the four-pole coils separates side beams away from each other, thereby canceling the effect that the side beams are shifted closer to each other after the electron beams enter the main magnetic field. This is synchronized with vertical deflection so as to generate the four-pole magnetic field with a strength in proportion to the vertical deflection current. In order to generate the four-pole magnetic field that has the same polarity regardless of a deflection polarity, a rectifier circuit of a diode is added.
On the other hand, the pincushion distortion at the left and right is corrected by means of a pincushion distortion correction circuit in the monitor set.
Next, in order to improve a focus performance, it is considered that a pincushion magnetic field of a horizontal deflection magnetic field is weakened so as to reduce the distortion of shapes of electron beam spots or to reduce the difference in shapes of spots of three electron beams. However, this method has the problem that a barrel-shaped residual mis-convergence of a vertical line mis-convergence on the horizontal axis (XH) occurs in a self-convergence system of in-line color display tubes, as shown in FIG. 11.
In order to solve this problem, as shown in FIG. 12, the invention described in JP 63-94542 A uses a circuit in which a four-pole coil 51 provided to a deflection yoke on the electron gun side is laid out as a bridge over four coils 52, 53, 54 and 55 of a saturable reactor. A horizontal deflection current is sent to this circuit so that the four-pole magnetic field that has the same polarity regardless of a polarity of the deflection current is generated with a strength being in proportion to the horizontal deflection current, thereby correcting beam spots. Since the XH is made variable at the same time, this method also can achieve an XH correction. In this circuit, the saturable reactor reduces inductance of the coils 52 and 55 in FIG. 12 during a positive half cycle (left deflection) and reduces inductance of the coils 53 and 54 during a negative half cycle (right deflection). Therefore, in any case, a horizontal deflection current runs through the four-pole coil 51 in the downward direction in the figure, generating a magnetic field having the same polarity. Such a relatively simple configuration can correct the XH.
However, as described above, when magnetic fields generated by the magnets for correcting the pincushion distortion at the top and bottom are strengthened, the increase in a YH correction amount is caused. If this correction of the increased amount is performed by means of a conventional YH correction, the YH correction amounts at the corners are larger than that on the vertical axis. Therefore, there has been a problem in that, even when the YH on the vertical axis is corrected optimally, a red beam right pattern of vertical line mis-convergence between side beams (PQH) occurs at the corners, as shown in FIG. 13.
In order not to increase the YH correction amount, it can be considered that barrel distortion of a vertical magnetic field of the deflection yoke is strengthened so as to reduce the pincushion-shaped residual mis-convergence. However, since this method leads to an increase in the pincushion distortion at the top and bottom again, there has been no alternative but to adopt a setting compromising with a PQH red beam right pattern.
On the other hand, with respect to the increase in the pincushion distortion at the left and right along with the development of flat panels, there has been no means but to increase a correction amount of the pincushion distortion of a monitor set, leading to an inevitable increase in power consumption accompanied by the increase in the correction amount.